1. Technical Field of the Invention
The present invention relates to dynamic random access memories, that is to say those requiring periodic refreshing of the data contained in the memory cells of these memories. Also, the invention relates more particularly to the process of refreshing these dynamic random access memories.
The invention applies advantageously, but without limitation, to cellular mobile telephones which incorporate dynamic random access memories.
2. Description of Related Art
Third-generation cellular mobile telephones will require the integration of large quantities of memory. However, the cost of the product must remain low. The use of dynamic random access memories (DRAMs), in place of the static random access memories (SRAMs) as are currently used, allows for this rise in memory capacity to be achieved at a low cost.
However, an important constraint in this type of application is the low electrical consumption while the telephone is on standby, so as not to discharge the batteries too quickly. However, although dynamic random access memories have a smaller static leakage current than static random access memories, they need to be refreshed continuously if the data has to be preserved in standby mode. This refreshing requires energy consumption that it is important to minimize.
The refresh frequency is given by the number of memory pages to be refreshed and by the retention time of the memory. This retention time is essentially related to the junction leakages of the transistors of the memory cells. Now, these leakages vary essentially as a function of temperature.
At present, dynamic random access memories are refreshed at the frequency corresponding to the worst operating case, that is to say to the maximum temperature of the application.
The article by Jae-Yoon Sim et al., entitled “Double Boosting Pump, Hybrid Current Sense Amplifier, and Binary Weighted Temperature Sensor Adjustment Schemes for 1.8 V 128 Mb Mobile DRAMs,” 2002 Symposium on VLSI Circuits Digest of Technical Papers, describes a temperature measurement system integrated onto the chip supporting the dynamic random access memory, and which acts on the refresh frequency. However, this system requires a tailoring of the temperature sensor at the on-board test level, this possibly complicating this test and therefore making it more expensive. Furthermore, a little current is consumed by the temperature sensor itself.
There is a need to optimize the refresh frequency of a dynamic random access memory, and to do so in a manner which is particularly simple to implement and without using an external sensor.